Convert fixed rig monohull mast to rotating multihull rig?

[Dayneger]

I have a mast and boom from an Impulse 21 that I'd like to convert to a rotating rig for a used trimaran a purchase a while back. Any thoughts on the viability of doing so, and how to achieve it?

The Impulse mast is 26' tall, with a single spreader, lower stays and a backstay. What I'm thinking--in theory at least:

- use the existing spreader to create a diamond stay (need to create attachment points low on mast)
- add attachment point just above the old forestay to provide rotating point for new forestay and backswept side stays (probably run to a bridle that bisects the tri's 2 cross beams. 17' of beam to work with).
- find an existing ball joint system to attach under the existing mast plate

Would this be safe? Effective?

It's probably not the ideal solution, but I do already have a really nice main and jib for it, the current mast is a scary noodle , and I'm trying not to throw significant money at the tri for a year or two!

 

[Dayneger]

Here's the Impulse drawing:

And here's an image of the 22' home designed/built trimaran (no, I don't plan on keeping the pseudo wishbone setup):

 

[rwatson]

It will work like a charm. Go for it.

 

[gggGuest]

So, if I understand you correctly, you have a good set of sails well cut for the mast set up as on the monohull, ie fixed, and so you want to change the mast to rotating? Got to ask why you want to change the mast setup so the sails don't match the mast any more? You'd surely be better keeping the rig as close to design as possible.

 

[tom28571]

No doubt you could fit a workable rotating mast rig from these parts. I have some reservations as to how well it would work though. From some experience it is clear that a rotating mast needs much support to prevent it from getting out of column as well as twisting and the single long pin ended spreader will not be sufficient. At least double diamonds with fixed (triangular) bases will be needed. Sides stays will need to be fixed to the nose of the mast to allow free rotation and that further induces twist and bend.

There are other issues but the main one is that rotating such a small mast section will not give the benefits of a more aerodynamic shape with a longer chord.

 

[rwatson]

.... You'd surely be better keeping the rig as close to design as possible.

Its a ""home design", not a professional design.

Its a lot better design than my effort
http://www.boatdesign.net/forums/projects-proposals/historic-home-made-tri-41690.html and I did a lot of rig experimentation on my slightly smaller and less robust project without major issues.

...... and this 26 ft mast should work fine.


If the original mast design had a single diamond spreader to keep it up, then that will still work here as well. Whatever boat it came off will have had the engineeering worked out properly. A mutlihull does put bigger loads on a mast, so you may need to slowly work up to heavy weather work.

The effectiveness of the rig is one issue, but getting any mast to rotate easily is a problem in itself. The leading edge mast fitting is usually a ring fitting, and with the forestay pulling the mast straight ahead, and the side stays confusing the issue, all the rotating masts I have come across were a big pain.

 

[tom28571]

If the original mast design had a single diamond spreader to keep it up, then that will still work here as well. Whatever boat it came off will have had the engineeering worked out properly. A mutlihull does put bigger loads on a mast, so you may need to slowly work up to heavy weather work.

Force vectors on a rotating mast are not the same as a normal 3 stay rig. There is no appreciable unresisted twisting force on a non rotating mast so simple diamonds will be OK. With both foot ball and forestay attached to the nose of the mast sail tension will exert a twisting force on a rotating mast and simple pin ended diamond spreaders will go out of plane and fail and the mast can come down. Not conjecture, seen it happen from just this cause. Take your own chances as you wish.

 

[Dayneger]

Thanks for all the feedback thus far, and you've made some good points.

In addition, I read a bit of general information about rotating rigs and apparently the sails are actually cut flatter than those for fixed rigs (and recutting the main isn't in the budget). The efficiency gain I would in theory get from rotating this particular, not rotating-optimized mast might end up being negated by the sail then being too deep. Since I sail where there's frequently a lot of wind, the latter would likely not be a good thing.

So, since I'm viewing this as a transition period for 2 or so years before putting a "real" rotating rig and matching sails on the boat, it would probably be a lot easier (and possibly safer) to just mount this rig fixed and use it as is. I have the mast mounting plate, simple to put on the boat. Not sure what this would mean for the standing rigging, though?

- Would I mount the lower stays to a similar location to where they were on the Impulse, or set them wider since I have lots of beam to play with?

- Since I'd like to skip the backstay, I could take the uppers out and back to a bridle between the two cross beams as planned for the rotating rig, in which case I wouldn't use the spreader, right? Or should I try to duplicate the layout of the Impulse 1:1 and make basically running backstays to that bridle?

I might be able to fit a backstay if needed, but the main is fairly high roach and requires a masthead whip, which I'd rather avoid if possible (the main snagging with every tack gets annoying after a while, and on some days I have to tack a lot to reach the Bay).

 

[gggGuest]

Its worth noting that in practice the efficiency gains from a rotating rig do not seem to be very great. There was a great enthusiasm for them back in the day, but it was based on rather simplistic aerodynamic theory. Now there's a deeper understanding it turns out that the negative effects of a fixed spar are far less than everyone thought they would be.

 

[Dayneger]

It would indeed be very interesting to see on the water comparisons of identical boats with rotating and fixed rigs, with the same total sail/surface area, and sails that are optimized for their respective rigs.

I've read about rotating rigs being centered, and how much better the boat performed when rotated, but that wouldn't be apples for apples since the sails would be cut much too flat for the centered position, resulting in the big depowering others have mentioned (which does sound like a very handy tool, much like cranking on backstay).

With more time spent thinking about it, I've decided to go ahead and keep the mast fixed since that's how my donor boat had it and I don't have time to mess with it. It should still be a major improvement over the useless noodle that came with the trimaran!

 

[Gary Baigent]

If you're serious about a rotating rig then you have to toss old world fixed rig compromises and start from scratch - and since you'll be aiming at better mast/sail aerodynamics, you need to go further - and that is, design and build a proper wing mast, not some piddling small chord mickey mousey thing with incorrectly shaped (meaning too full) earlier fixed mast sails. You can't be a little bit pregnant in this situation; either you retain archaic thinking or bite the bullet and totally redesign for a modern decent chord mast with square topped mainsail rig. And you don't use your antique backstay fixed rig setup either but will have to go to three quarter positioned runners (or maybe masthead runners if you insist). Oh, and since you're going to a larger chord and thickness mast, you can toss all the dopey bits of spreaders etc. rigging, which will give you less clutter and better aerodynamics. Obviously I've poured defecation over your suggestion - but as said, being a little bit pregnant doesn't work
On a positive note, if you do decide to go for a modern rig, then your boat performance will obviously improve by a decent percentage. And to put money where mouth is, here's the rig on my Cox's Bay Skimmer.

 

[Dayneger]

Now that I've looked into it further and heard comments from others, I agree with you that a rotating rig would only really make sense if the whole system were designed from ground up to perform optimally in that configuration.

So, I'm glad I asked first.

You've obviously lavished a great deal of time and thought into your Cox's Bay Skimmer. Why so much effort to make a light monohull viable when you could've let the beam of a multihull do the work?

No, not trying to hijack my own thread, just couldn't help wonder out loud about that low CE split rig.

Thanks for the input!

 

[Gary Baigent]

I designed and built the Skimmer for a friend several, probably more, years ago and after he became too ill and too old to sail it he gave it to me - and I sailed it on and off for three years, then gave it away to a Waiheke Island enthusiast who eventually gave it back to me after it got damaged. I thought it too good a boat to let die so repaired and altered it during this last month.
The low CoE, but very large sail area and wing masts for an 18 foot boat is intentional, I wanted power but a low rig. The wing masts also make the Skimmer very close winded, a bonus.
Yes, multihulls make more sense and I have three of them, my own designs; here's a couple of images of Sid, 8.5 x 8 metres.